The present invention relates to a system for transmitting electric power. A direct voltage network is connected to at least two alternating voltage networks through a respective power station. The power stations transmit electric power between the direct voltage network and the respective alternating voltage network. Each station includes at least one VSC-converter adapted to convert direct voltage to alternating voltage, and conversely, to convert alternating voltage to direct voltage. An apparatus of a first station regulates the direct voltage of the direct voltage network at the first station to a predetermined nominal value. The station detects the occurrence of a predetermined unbalanced state at the converter of the second station, and upon such detection, controls a switching member to connect the direct voltage network at the first station through a resistor to ground for shunting electric power and thereby restore the direct voltage of the network to the nominal value.
Such a system has recently become known through the thesis "PWM and Control of Two and Three Level High Power Voltage Source Converters" by Anders Lindberg, Kungliga Tekniska Hogskolan, Stockholm, 1995. The publication describes a system for transmitting electric power through a direct voltage network for High Voltage Direct Current (HVDC). It points out that the invention is not restricted to this application, but for purposes of describing t he invention, the application of the invention to systems of the type defined above is illustrative.
Prior to the issuance of the thesis, facilities for transmitting high voltage direct current through a direct voltage network used a line-coummutated CSC (Current Source Converter) converter in power transmission stations. The development of IGBTs (Insulated Gate Bipolar Transistor, i.e., a bipolar transistor having an insulated gate) has resulted in a circuit component for high voltage applications which is suitable for connecting in series to form valves in converters, since they may easily be turned on and turned off. These VSC (Voltage Source Converter) converters for forced commutation are now an alternative to the commutated CSCs for transmission of electric power between a direct voltage network for supplying High Voltage Direct Current, and alternating voltage networks. They offer several important advantages with respect to line-commutated CSCs in HVDC, including control of active and reactive power flow independently of each other without the risk of commutation failures in the converter, or the risk of transmission of commutation failures between different HVDC links which may take place in a line-commutated CSC. Furthermore, there is the possibility of feeding a weak alternating voltage network or a network that does not generate its own power (a "dead" alternating voltage network) as well as other advantages.
In a plant of the type which converts a direct voltage network to two alternating voltage networks using VSC-converters, the direct voltage of the direct voltage network is determined by one of its power stations. It is desirable to manage a power unbalance, i.e., when the active power fed into the direct voltage network differs from the active power fed out from the direct voltage network, without any rapid telecommunication between the station with the voltage regulating apparatus and the other stations along the direct voltage network. The most probable unbalance occurs when a station is blocked out of operation as a protective measure. Blocking may have many different causes, such as an occurrence of an over-voltage or an over current condition, of the station, or a detected internal failure. If the flow of active power before the blocking of the station was from the direct voltage network to the alternating voltage network, the direct voltage will increase after the converter of the station is blocked.
To limit the resulting over-voltage occurring in the direct voltage network, switching members connect the direct voltage network connected to the station through a resistor to ground for shunting or draining electric power from the direct voltage network. The voltage regulating station detects the over-voltage occurring at the direct voltage network, and in response thereto, regulates the voltage by changing the active current orders thereof, so that power fed into the direct voltage network is reduced, or the direction of power transfer is reversed, until the nominal voltage and balance is achieved. A disadvantage of a system of this type is that a voltage regulating station has to detect an over-voltage of the direct voltage network before it takes care of the power unbalance problem. This means that an over-voltage has time to occur in the entire direct voltage network with the risk that other converters will block as a consequence of this over-voltage, resulting in an even higher voltage on the direct voltage network, disabling further converters and accelerating the failure of the stations. Such failures may have very severe consequences.